Peter H. Trane, Skamol A/S, Denmark, explores acid resistance in preheaters that utilise alternative fuels
POROS 500 and HIPOR 450 allow cement plant operators to significantly reduce heat loss and surface temperature without compromising the insulating lining. This enables the construction of low-weight refractory linings with excellent thermal efficiency, minimising the overall refractory load while maintaining the temperatures needed for those hard-to-burn alternative fuels.
In modern cement kilns, the sintering temperature is achieved by burning various fuels inside the rotary kiln and precalciners. Today’s precalciner technology has allowed for lower heat loading, larger diameter and reduced length of the rotary kiln, which significantly increases clinker production. In recent years, low grade alternative fuels have steadily made their way into cement cyclone precalciners due to the low combustion temperature found there. The use of alternative fuels in precalciners are, however, causing significant problems in the form of build-up and chemical/alkali attacks while the heavy load of the preheater tower itself causes risk of civil construction failure. As a consequence, Skamol introduced the POROS 500 and HIPOR 450, which since then have become the preferred alternatives for backup insulation solutions for all parts of the cement cyclone preheater and pyroprocessing tower which risk exposure of acid attacks.
With increasing fuel prices and attention to CO2 emissions, the industry is more and more interested in the combustion of alternative fuels. The net benefit if you replace coal with solvent waste is 967 kg/CO2/t of waste1 compared to incineration. Especially in regions with carbon emission tax or where tax incentives are rewarded for carbon emission reduction; this alone can constitute significant savings, not to mention the savings in fuel costs. The downside of the combustion of alternative fuels such as waste, rubber, nut shells and other materials is that the plant often experiences more problems with corrosion in the coolers and preheaters than with conventional fuels.
With conventional fuels, the flue gas consists of a mix of carbon dioxide (CO2), water vapor (H2O), nitrogen (N2) and excess of oxygen (O2). Minor contents of carbon monoxide (CO), nitrogen oxides (NOx), sulphur oxides (SO2 and SO3) can be present too. Together with the water vapor, the sulphur oxides can form sulphur acid which in itself can cause problems when the acid condensates. In addition, when you combust polymers containing chloride, e.g. tires and waste, the flue gases will also contain hydrochloric acid and nitric acid. The condensate of these acids increases the corrosion on the steel shell.
The acid attacks occur where the temperature in the flue gas within the insulation or refractory is at a specific acid dew point. Bricks based on diatomaceous earth are characterised by having a high content of silica and a low content of acid soluble oxides such as ferric oxide, potassium oxide and sodium oxide. These properties give excellent protection against acid attacks.
The Skamol products produced in Russia are made from diatomaceous earth from a deposit that has a very low content of acid soluble oxides. The average composition can be seen in Table 1.
Russian diatomite bricks
Danish diatomite bricks
|Oxide name||Content in weight %||Content in weight %|
|Fe2O3 (Ferric oxide)||2.8||7.0|
|CaO (Calcium oxide)||0.3||0.8|
|K2O (Potassium oxide)||0.2||1.6|
|Na2O (Sodium oxide)||1.3||0.4|
|Loss On Ignition (LOI), 1025o C||0.7||1.0|
Calculations2 show that the dew point temperature for sulphuric acid is in the range from 150° C to 175° C depending on the water and sulphur trioxide content in the flue gas. For nitric acid the dew point temperature is from 40° C to 65° C, and for hydrochloric acid it is from 50° C to 68° C.
To avoid corrosion of the steel shell, the optimum solution will be to design the refractory layer with the dew point temperature inside the diatomite brick, or alternatively with the dew point temperature “outside” the steel shell (the acid gas will not condensate inside the construction).
For example, in Figure 1 the temperature of the external wall is calculated to 80° C. This temperature is above the dew point temperature for nitric and hydrochloric acid, and below the dew point temperature of sulphuric acid.
When considering the refractory lining type in an area with possible acid attacks in the refractory or at the shell, special care must be taken in order to find the optimum solution. This applies both to the selection of hot face refractory and insulation. Focus on chemical analysis and thermal calculations is important. When conducting the analysis of the thermal calculations, it is important that they include both a new lining and a worn lining, as the temperature in the boundary layer between the refractory and the insulation changes as the hot face refractory is worn. It is advisable to maintain an external shell temperature above 70° C or below 40° C in order to avoid acid attacks from nitric and hydrochloric acids on the steel shell. Likewise, it is recommended to design the lining in such a manner that the hydrochloric acid condensates inside the layer of POROS 500 or HIPOR 450 brick, which will prolong the refractory lining and reduce corrosion problems.
Consequently, the acidic corrosion of the steel shell, will be theoretically non-existing, as the dew point temperature of sulphuric gas is inside the diatomaceous brick, and the temperature is too high for condensing nitric and hydrochloric gasses.
Due to the composition of POROS 500 and HIPOR 450 the physical properties does not change significantly even after all oxides in the bricks have been dissolved. This protects both the integrity of the lining and the thermal balance in the affected areas.
To avoid failure, focus should be towards correct anchoring. Risk of failure of refractory and insulation linings could also be due to acid condensation dew point on anchoring system. Therefore, it is important that linings are designed with low risk for permeability of acid gasses through expansion joints, dry-out shrinking of refractory castables or gaps between hot-face and insulation where flue gas flow could occur (chimney effect). All these examples constitute potential dew points for acid gasses, because cooling of steel anchors combined with open structures in between layers cause acid gasses to condensate.
Making the right anchor design is not an easy task and requires many years of experience and knowledge about the specific process area; e.g. temperatures, flue gas turbulence, expansion of materials, wear on hot-face lining, etc. The same applies for insulation linings. They are also placed in areas with acid gasses and other chemical attacks. Insulation materials must continue to absorb movements throughout the lifetime and help to protect anchoring systems against corrosion. It is important that insulation materials have some acid resistance in combination with a hard surface, so dislocations between each material layer will not affect the insulation lining lifetime due to wear or/and mechanical stress.
All designs must be developed in such a way that acid condensation is reduced as much as possible. If that is not possible, a coating solution could be combined with the right design. Especially around weldings of anchors, because melt baths may be contaminated with impurities which constitutes a major risk for corrosion. Applying a coating on the specific areas can reduce - but not completely eliminate - corrosion speed. To reduce corrosion it is important to focus on the design of the refractory, insulation and anchoring.
In lining systems with preheaters fueled by alternative fuels, acid corrosion can be minimised by selecting the right anchor system and using acid resistant material. By using a lining with POROS 500 or HIPOR 450 you can exclude the anchor system, consequently the typical points of acid attacks, the anchor weldings, are eliminated from the system. The lifetime of the refractory lining is, therefore, not dependent on the corrosion of refractory anchors and the possibility of civil construction failure in the lining, but is mainly dependent of the abrasion resistance of the refractory. The anchors will normally act as a thermal bridge, thereby increasing the temperature on the steel casing. Localised emergency repair is also significantly easier with a brick lining. A further advantage is that there is no drying time of the insulating layer and therefore the start up time can be minimised, and problems with water in the process during start up can likewise be avoided.
2 Refractories for Energy Generation - Acid resistant insulation solutions, Peter Herman Trane, Institute of Refractories Engineers National Conference 2014, Sheffield, UK